Dispersion stability and microstructural transition of colloidal silica suspensions were examined by rheological measurements under either steady simple shear or oscillatory flow. Monodisperse silica particles were prepared by the so-called modified Stober method and were stabilized by either steric or electrostatic repulsive force. Depending upon the methods of stabilization, the suspension showed hard-sphere or soft-sphere response. In particular, silica suspensions exhibited hardsphere response when the silica spheres coated with 3-(trimethoxysilyl)propyl methacrylate (MPTS;CH₃O)₃Si(CH₂)₃OCOC(CH₃)=CH₂) were dispersed in a refractive-index matching solvent, tetrahydrofurfuryl alcohol. On the other hand, silica particles in aqueous media behaved like soft spheres with long-range elec-trostatic repulsive interactions when they were coated with steric layer of aminosilane coupling agent, N-[3-(trimethoxysilyl)propyl]ethylenediamine((CH₃O)₃Si(CH₂)₃NHCH₂CH₂NH₂). In this case, the electrostatic repulsion or equivalently the softness of the silica spheres was contorlled by the ionic strength using a symmetric salt KCI. Both the hardsphere and soft-sphere suspensions showed stable shear-thinning behavior without experiencing shear-induced flocculation. Moreover, the oscillatory shear rheology. showed that the electrostatically stabilized soft-sphere suspensions underwent a microstuctural transition from liquid-like to solid-like structure when either the particle loading increased or the ionic strength was recuced.